A new type of SRAM device using Negative Differential Resistance Field Effect Transistors (NDR FETs) is described in detail in a patent application Ser. No. 10/029,077 filed Dec. 21, 2001 by T J King and assigned to the present assignee, and published on May 9, 2002 as Publication No. 2002/0054502. The NDR FET structure, operation and method of making the same are discussed in detail in patent application Ser. No. 09/603,101 filed Jun. 22, 2000 by King et al., which is also assigned to the present assignee. Such details are also disclosed in a corresponding PCT application PCT/US01/19825 which was published as publication no. WO 01/99153 on Dec. 27, 2001. The above materials are hereby incorporated by reference.
As is well-known in the art, the “body effect” refers to a phenomenon in which a threshold voltage of a transistor is increased substantially when a body-to-source pn junction is reverse-biased. This effect can also influence SRAM embodiments which use NDR devices. For instance in the SRAM embodiment of FIG. 1, the body effect causes Vt of “pull-up” NDR FET 130 to be increased when data stored at node Vsn is a high value (Vdd); in such instances, a relatively high gate bias (Vdd+Vt) is required then to switch “pull-up” NDR FET 130.
Thus there is clearly a need for NDR based SRAM device that have superior body effect characteristics.